Optical component and method for producing the same

ABSTRACT

A concave lens as an optical component is made of glass containing 20 to 22% of B 2 O 3 , 30 to 40% of La 2 O 3 , and 19 to 25% of ZnO, expressed as wt %. The concave lens has a thickness t 1  in its center portion of 0.5 mm or less, and a ratio (W/t 1 ) of a diameter W with respect to the thickness t 1  of 24 or more. The concave lens can be produced suitably by press forming.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical component and a method forproducing the optical component.

2. Description of Related Art

Glass lenses to be used for DSC (digital still camera) or DVC (digitalvideo camera) are produced by press forming. Glass lenses having anaspherical shape can be produced at low cost by press forming.

In order to produce lenses with high shape accuracy, it is important toprevent glass from fusion bonding to a forming die, and the forming dieis required to have very high surface accuracy and low surfaceroughness, in press forming. The following techniques are proposed forimproving the glass (formed lens) separation from a forming die. JP 62(1987)-207728 A discloses a technique in which the surface of a glasssubstrate is provided with a surface layer having a lower concentrationof easily volatile components than the glass main body so that highsurface accuracy and low surface roughness of a forming die can beensured. JP 8 (1996)-217468 A discloses a technique for preventing glassfrom fusion bonding to a forming die by forming a carbon film on thesurface of a glass preform.

It should be noted that the term “fusion bonding” in this descriptionmeans a phenomenon in which a part of glass adheres to a forming die soas to remain as a residue on the surface of the forming die, or aphenomenon in which the formed product (lens) of glass sticks to theforming die. In forming a thin lens, either phenomenon can occur.

In recent years, the demand for reduction in size of image pickupoptical systems represented by DSC and DVC has been increasing more andmore. In order to reduce the size of image pickup optical systems, it isnecessary to reduce the lens thickness. Specifically, it is necessary toestablish a technique for mass producing very thin lenses such asconcave lenses having a thickness of 0.5 mm or less in its centerportion and a convex lens having a thickness of 0.5 mm or less in itsouter peripheral portion (edge portion).

However, it is very difficult to produce such a thin lens byconventional press forming methods. In forming a lens, load isconcentrated on the thin portion of the lens, thus causing glass to befusion bonded to a forming die notably. The fusion bonding of glass tothe forming die not only deteriorates the shape accuracy of a lens as afinished product, but also impairs the surface accuracy of the formingdie because of the residual glass adhering to the forming die, whichmakes it difficult to use the same forming die continuously. Althoughthe techniques disclosed in JP 62 (1987)-207728 A and JP 8 (1996)-217468A have some effect in preventing the glass from fusion bonding to theforming die, the effect is still insufficient. Further, the techniquesof JP 62 (1987)-207728 A and JP 8 (1996)-217468 A suffer from problemssuch as an increase in the number of steps, and an increase in cost,because various pretreatments are required therein.

SUMMARY OF THE INVENTION

It is an object of the present invention to shed light on a glasscomposition suitable for a thin lens, and to provide a lens (opticalcomponent) produced using the glass.

That is, the present invention provides an optical component made ofglass containing 20 to 22% of B₂O₃, 30 to 40% of La₂O₃, and 19 to 25% ofZnO, expressed as wt %. The optical component is formed as a concavelens that has a thickness t₁ in its center portion of 0.5 mm or less anda ratio (W/t₁) of a diameter W with respect to the thickness t₁ of 24 ormore, or a convex lens that has a maximum thickness in its outerperipheral portion of 0.5 mm or less.

According to another aspect of the present invention, there is provideda method for producing an optical component including the steps of:supplying a preformed body made of glass for an optical component into aforming die; closing the forming die; transferring the surface shape ofthe forming die to the preformed body by heating and pressing thepreformed body; and opening the forming die in order to remove theformed optical component therefrom. The glass for the optical componentcontains 20 to 22% of B₂O₃, 30 to 40% of La₂O₃, and 19 to 25% of ZnO,expressed as wt %, and the optical component is formed as a concave lensthat has a thickness t₁ in its center portion of 0.5 mm or less and aratio (W/t₁) of a diameter W with respect to the thickness t₁ of thecenter portion of 24 or more, or a convex lens that has a maximumthickness in its outer peripheral portion of 0.5 mm or less.

The inventors have found that there is a correlation between thelikelihood of the occurrence of glass fusion bonding to a forming dieand the composition of the glass. That is, use of the glass containingcomponents within the range as mentioned above can prevent the glassfrom fusion bonding to a forming die, so that stable production of thinlenses by press forming is rendered possible. The present inventioneliminates the need for particular pretreatments also, and thus is freefrom the problems such as an increase in the number of steps and anincrease in cost. The lens provided by the present invention enables areduction in the size of image pickup optical systems such as DSC.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a top view and a sectional view illustrating an opticalcomponent (concave lens) according to one embodiment of the presentinvention.

FIG. 2 shows a top view and a sectional view illustrating an opticalcomponent (convex lens) according to another embodiment of the presentinvention.

FIGS. 3A to 3D are views illustrating the steps of the method forproducing a lens.

DETAILED DESCRIPTION OF THE INVENTION

As indicated in FIG. 1, a lens 1 according to one embodiment of thepresent invention is formed as a concave lens having a concave surface 1a and a convex surface 1 b. The lens 1 is a so-called aspherical lensand has a meniscus shape. The lens having a meniscus shape means acrescent shaped lens with one surface being convex and the other surfacebeing concave. Types of meniscus lenses include a concave meniscus lensin which the outer peripheral portion has a thickness larger than thecenter portion, and a convex meniscus lens in which the center portionhas a thickness larger than the outer peripheral portion. The lens 1indicated in FIG. 1 is a concave meniscus lens.

The lens 1 is suitable for small optical devices, and has a diameter W,for example, in the range of 10 to 20 mm. The center portion of the lens1 has a thickness t₁, for example, in the range of 0.1 to 0.5 mm. Theratio (W/t₁) of the diameter W with respect to the thickness t₁ of thecenter portion is, for example, 24 or more. The upper limit of the ratio(W/t₁) is not limited particularly, but for example, is 200.

As indicated in FIG. 2, a lens 2 according to another embodiment of thepresent invention is formed as a biconvex lens having two convexsurfaces 2 a and 2 b. The lens 2 also is a so-called aspherical lens.The lens 2 further has an outer peripheral portion 2 c (edge portion)with a reduced thickness. The outer peripheral portion 2 c is a portionthat is used for fixing the lens 2 to a housing in an optical device,and that does not form an optical path. As is the lens 1, the lens 2also is suitable for small optical devices, and has a diameter W, forexample, in the range of 10 to 20 mm. The outer peripheral portion 2 cof the lens 2 has a maximum thickness t₂, for example, in the range of0.1 to 0.5 mm.

Next, the method for producing the lens 1 is described. The productionmethod according to this embodiment employs a press forming method(so-called precision glass forming) using a forming die. First, apreformed body to be used for producing the lens 1 is prepared.Specifically, the preformed body is obtained by grinding and/orpolishing optical glass with a particular composition to process it intoan analogous shape to the lens 1.

In the case of producing a thin lens by press forming, it is importantto prevent glass from fusion bonding to the forming die. As seen fromthe later mentioned examples, there is a correlation between thelikelihood of the occurrence of the glass fusion bonding to the formingdie and the composition of the glass. That is, by using the glasscontaining 20 to 22% of B₂O₃, 30 to 40% of La₂O₃, and 19 to 25% of ZnO,expressed as wt %, it is possible to prevent the fusion bonding of theglass to the forming die effectively.

Further, the glass to be used for producing the lens 1 (or the lens 2)has a refractive index (nd) in the range of 1.79 to 1.83 and an Abbenumber (νd) in the range of 39 to 43, for example. The glass having ahigh refractive index is advantageous in reducing the thickness of thelens. The glass having a high Abbe number enables clear images to beobtained.

A detailed example of the glass composition that satisfies therequirements mentioned above is described below. The values areexpressed as wt %.

SiO₂: 3 to 6%, preferably 5 to 6%B₂O₃: 20 to 22%, preferably 20 to 21%Li₂O: 0 to 2%, preferably 0 to 1%ZnO: 19 to 25%, preferably 20 to 24%ZrO₂: 3 to 5%, preferably 3 to 4%TiO₂: 0 to 2%, preferably 0 to 1%Nb₂O₅: 2 to 7%, preferably 5 to 7%La₂O₃: 30 to 40%, preferably 34 to 40%Ta₂O₃: 0 to 10%, preferably 0 to 4%WO₃: 0 to 7%, preferably 0 to 2%

Next, as indicated in FIGS. 3A to 3D, a preformed body 10 is pressformed. A forming device provided with an upper die 3, a lower die 4, acylindrical die 5, an upper head 6 and a lower head 7 can be used forpress forming. In the forming device, the upper head 6 is provided abovethe upper die 3, and the lower head 7 is provided so as to support thelower die 4. The upper head 6 is provided with heating and pressingmechanisms (not shown in the drawings). Similarly, the lower head 7 isprovided with a heating mechanism.

The upper die 3 includes a convex portion 3 a and a supporting portion 3b that surrounds the convex portion 3 a. The convex portion 3 a is aportion in contact with the concave surface of the preformed body 10,and the supporting portion 3 b is a circular portion in contact with theouter peripheral portion of the preformed body 10. Each surface of theupper die 3 and the lower die 4 is processed to have a desired accuracyand shape so that the lens 1 to be shaped by the upper die 3, the lowerdie 4 and the cylindrical die 5 has optical properties as designed.Depending on the circumstances, the surface of the cylindrical die 5also is processed to have a desired accuracy and shape.

The upper die 3, the lower die 4 and the cylindrical die 5 typically aremade of cemented carbide such as WC. The cemented carbide is preferableas a material of forming dies because of its excellent strength. Theupper die 3, the lower die 4 and the cylindrical die 5 each may beconstituted by a base material made of metal such as stainless steel anda plating layer (for example, electroless nickel plating layer) coveringthe surface of the base material. Furthermore, a releasing layer may beformed on each surface of the upper die 3, the lower die 4 and thecylindrical die 5 for improving the releasability and the corrosionresistance.

First, the preformed body 10 is supplied into a forming die that isconstituted by the upper die 3, the lower die 4 and the cylindrical die5 as indicated in FIG. 3A.

Next, the upper head 6 is lowered, so that the preformed body 10 isclamped by the upper die 3, the lower die 4 and the cylindrical die 5 asindicated in FIG. 3B. Then, the heating mechanisms in the upper head 6and the lower head 7 are activated in this state to heat the preformedbody 10 to a predetermined temperature. The predetermined temperatureis, for example, a slightly higher temperature than the glass transitiontemperature (Tg) or the yield temperature (At) of the glass to be used(for example, a temperature higher than the Tg or At by 20 to 40° C.).Preferably, the heating is performed at an approximate temperatureincrease rate in which the preformed body 10 takes several minutes toreach the predetermined temperature (for example, 5 minutes).

After the preformed body 10 has reached the predetermined temperature,the upper head 6 is lowered slowly and thereby pressure is applied onthe upper die 3, as indicated in FIG. 3C. The pressure from the upperhead 6 is applied on the preformed body 10 via the upper die 3, so thatthe preformed body 10 is deformed gradually. Thus, the shape of eachsurface of the upper die 3 and the lower die 4 is transferred to thepreformed body 10. The pressure to be applied on the preformed body 10is, for example, 50 to 300 kgf.

When the preformed body 10 is deformed into the shape of the lens 1, theupper head 6 stops applying the pressure. Then, this state is maintainedat the above-mentioned predetermined temperature for about 1 minute.Thereafter, the heating mechanisms in the upper head 6 and the lowerhead 7 are turned off, and the lens 1 is cooled to the glass transitiontemperature while the upper head 6 is kept lowered. In order to avoidthe occurrence of defects in the lens 1 such as distortion, it ispreferable that the lens 1 be cooled slowly over several minutes (forexample, about 5 minutes).

Finally, the upper head 6 and the upper die 3 are opened so that theformed lens 1 can be removed therefrom, as indicated in FIG. 3D. Itshould be noted that each step of FIGS. 3A to 3D can be performed underan inert atmosphere such as a nitrogen atmosphere or an argonatmosphere. Such shape forming under an inert atmosphere is preferablefor extending the life span of a forming die because the oxidation ofthe forming die can be prevented.

The concave meniscus lens 1 described with reference to FIG. 1 generallyhas the concave surface 1 a that is inclined more steeply thanconventional lenses such as a spherical lens and convex meniscus lens.Therefore, the glass is likely to be fusion bonded to a forming die.Accordingly, as is this embodiment, it is particularly effective toemploy glass having a composition that is unlikely to cause theoccurrence of fusion bonding as a material of the concave meniscus lens.Of course, such glass is significant also for a biconvex lens that is alens having two convex surfaces in combination (FIG. 2), a biconcavelens that is a lens having two concave surfaces in combination, and aplanoconvex lens and planoconcave lens each being a lens with one flatsurface.

EXAMPLES

Using glass having a composition and optical constant indicated in Table1, concave meniscus lenses having a shape indicated in FIG. 1 wereproduced by press forming that has been described with reference toFIGS. 3A to 3D. Specifically, using glass No. 1 to 3, 20 lenses for eachof 4 kinds of concave meniscus lenses that have a thickness t₁ in thecenter portion, respectively, of 0.9 mm, 0.7 mm, 0.5 mm and 0.3 mm wereproduced continuously. The presence or absence of fusion bonding of theglass to the forming die was investigated. All the lenses were set tohave a diameter W of 14 mm, an approximate concave curvature of 4 mm,and an approximate convex curvature of 280 mm. The press forming processwas performed under a nitrogen atmosphere. At the time of glassformation, the heating temperature was set to 600° C., and the pressureforce was set to 150 kgf. Table 1 indicates the results.

TABLE 1 GLASS No. 1 GLASS No. 2 GLASS No. 3 nd 1.81 1.81 1.81 νd 41 4141 SiO₂ 5.4 3.5 3.2 B₂O₃ 20.4 20.8 18.8 Li₂O 0.6 1.5 2.0 ZnO 22.6 20.919.7 ZrO₂ 3.3 3.2 4.2 TiO₂ 0.8 — — Nb₂O₅ 6.2 2.6 2.0 La₂O₃ 39.2 34.025.2 Gd₂O₃ — — 10.3 Ta₂O₃ — 9.9 7.9 WO₃ 1.6 3.7 6.7 FORMATION EVALUATION0.9 mm ∘ ∘ ∘ 0.7 mm ∘ ∘ x 0.5 mm ∘ ∘ x 0.3 mm ∘ Δ x

In Table 1, the formation was evaluated as “o” that means no fusionbonding occurred, “Δ” that means fusion bonding occurred in 1 to 3lenses, and “x” that means fusion bonding occurred in 4 or more lenses,when 20 lenses were produced continuously.

In the case of using the glass No. 1, even if the thickness of thecenter portion was reduced to 0.5 mm or less, the glass was not fusionbonded to the forming die, and stable formation of concave meniscuslenses having a desired shape was possible. In the case of using theglass No. 2, slight fusion bonding occurred when the center portion ofthe lens had a thickness of 0.3 mm, though favorable results wereobtained in general. In contrast, in the case of using the glass No. 3,the glass was fusion bonded to the forming die in a significantpercentage when the thickness of the center portion was 0.7 mm or less,and stable formation was not feasible.

As a result of X-ray photoelectron spectral analysis (XPS) for thesurface of each forming die after concave meniscus lenses were formedusing the glass No. 1 to No. 3, peaks based on B, La and Zn wereobserved. That is, B, La and Zn contained in the glass were present onthe surface of the forming die. The present amount of B, La and Zn waslarger in the order of the forming die used for the glass No. 1, theforming die used for the glass No. 2, and the forming die used for theglass No. 3. In view of these, it is conceivable that when the glasscontains an appropriate amount of B, La and Zn, these components cause areleasing effect by spreading or diffusing on the surface of the formingdie, thereby rendering the fusion bonding of the glass to the formingdie unlikely to occur.

From the above results, it can be said that use of glass that contains20 to 22% of B₂O₃, 30 to 40% of La₂O₃, 19 to 25% of ZnO, expressed as wt%, can prevent the glass from fusion bonding to a forming die. Thepreferable contents of B₂O₃, La₂O₃, and ZnO are respectively 20 to 21%,34 to 40%, and 20 to 24%. It may seem that the greater the containedamounts of these components, the more the prevention effect of thefusion bonding is enhanced, but when they are contained in an excessamount, there are problems such as the difficulty of achieving a desiredoptical constant and the possibility of causing the devitrification ofglass. Specifically, an excess content of B₂O₃ makes it difficult toadjust the refractive index to a desired range. An excess content ofLa₂O₃ causes glass devitrification to occur easily. An excess content ofZnO makes it difficult to adjust the refractive index and Abbe number toa desired range. Accordingly, each component should be contained withinthe above-mentioned range. By using the glass with such a composition,concave lenses that have a thickness t₁ in its center portion of 0.5 mmor less and a ratio (W/t₁) of a diameter W with respect to the thicknesst₁ of the center portion of 24 or more can be produced by press formingat a high yield. By preventing the glass fusion bonding to a formingdie, the surface accuracy of the forming die also can be kept high andthe surface roughness thereof can be kept low. This can reduce thefrequency of maintenance for a forming die, and as a result, it becomespossible to achieve high productivity.

It should be noted that tendencies similar to the case of the concavelens were observed in the case of forming the convex lens indicated inFIG. 2. That is, although the thinner the outer peripheral portion 2 cwas, the more the fusion bonding of glass to a forming die occurredeasily, stable formation of convex lenses having a maximum thickness t₂in its outer peripheral portion 2 c of 0.5 mm or less was possible byusing the glass No. 1 and No. 2.

The optical component (optical lens) of the present invention can beused suitably for optical devices such as DSC, DVC, cell phone cameras,projection televisions, optical pickups, and the like.

The present invention may be embodied in other forms without departingfrom the spirit or essential characteristics thereof. The embodimentsdescribed in this specification are to be considered in all respectsonly as illustrative and not restrictive. The scope of the invention is,therefore, indicated by the appended claims rather than by the foregoingdescription. All changes which come within the meaning and range ofequivalency of the claims are to be embraced within their scope.

1. An optical component made of glass containing 20 to 22% of B₂O₃, 30to 40% of La₂O₃, and 19 to 25% of ZnO, expressed as wt %, the opticalcomponent being formed as a concave lens that has a thickness t₁ in itscenter portion of 0.5 mm or less and a ratio (W/t₁) of a diameter W withrespect to the thickness t₁ of 24 or more, or a convex lens that has amaximum thickness in its outer peripheral portion of 0.5 mm or less. 2.The optical component according to claim 1, wherein the glass has arefractive index (nd) in the range of 1.79 to 1.83, and an Abbe number(νd) in the range of 39 to
 43. 3. The optical component according toclaim 1, wherein the optical component is produced by press forming. 4.A method for producing an optical component comprising the steps ofsupplying a preformed body made of glass for an optical component into aforming die; clamping the preformed body with the forming die;transferring a surface shape of the forming die to the preformed body byheating and pressing the preformed body; and opening the forming die inorder to remove the formed optical component therefrom, wherein theglass contains 20 to 22% of B₂O₃, 30 to 40% of La₂O₃, and 19 to 25% ofZnO, expressed as wt %, and the optical component is formed as a concavelens that has a thickness t₁ in its center portion of 0.5 mm or less anda ratio (W/t₁) of a diameter W with respect to the thickness t₁ of 24 ormore, or a convex lens that has a maximum thickness in its outerperipheral portion of 0.5 mm or less.